I believe the big change from 1.1 to 1.2 was adding Thermal Dynamics, although I could be wrong.  Also, most mods updated versions to their latest when this update kicked in.  Also Botania configs changed here to disallow shedding for a good number of mobs.
1.2.1 changed a few glaring glitches which allowed item duplication, infinite RF, and duplication of fluids.  I'm sure other things happened in that bump too, but that was a huge fix.
The core mods didn't change, so world generation shouldn't be affected.  Nothing that I know of will make your world explode if you update.